Apparatus for producing strong and highly opaque random fibrous webs

ABSTRACT

Apparatus for producing a randomly mixed fibrous web of high strength and opacity. A thermoplastic polymer is extruded through a die having slots of varying length and cross-sectional area. Air at about the polymer melt temperature is impinged angularly on the fibers and allowed to expand thus cooling the fibers and breaking them up into varying dimensions as they are deposited on a carrier. In a preferred embodiment a controlled Coanda effect is applied so as to provide an overall wavy pattern of randomly mixed fibers.

United States Patent [191 Schwarz APPARATUS FOR PRODUCING STRONG ANDHIGHLY OPAQUE RANDOM FIBROUS WEBS [75] V Inventor: Eckhard C. A.Schwarz, Neenah,

Wis.

[73] Assignee: Kimberly-Clark Corp., Neenah,

Wis.

[22] Filed: Apr-5, 1972 [211 Appl.No.:241,373

[52] US. Cl 425/72, 264/115, 425/461 [51] Int. Cl B29f 3/08 [58] Fieldof Search 425/72, 83, 461; 264/115,

[ 56] References Cited 1 UNITED STATES PATENTS 12/1953 12/1962 10/1970Stalego 264/121 X White et al. 264/121 X FOREIGN PATENTS OR APPLICATIONS3/1969 Great Britain 264/210 R Pukacz 264/121 x [4 1 Apr.23, 1974 61,1925/1948 Netherlands 425/72 437,193 11/1967 Switzerland 264/115 PrimaryExaminerR. Spencer Annear Attorney, Agent, or FirmDaniel J. I-lanlon,Jr.; William D. Herrick; Raymond J. Miller 57 ABSTRACT Apparatus forproducing a randomly mixed fibrous web of high strength and opacity. Athermoplastic polymer is extruded through a die having slots of varyinglength and cross-sectional area. Air at about the polymer melttemperature is impinged angularly on the fibers and allowed to expandthus cooling the fibers and breaking them up into varying dimensionsasthey are deposited on a carrier. In a preferred embodiment a controlledCoanda effect is applied so as to' provide an overall wavy pattern ofrandomly mixed fibers. v

7 Claims, 8 Drawing Figirres LINEAR v POLYMER H EAT TO EXTRUDABLE MELTEXTRUDE THROUGH SLOTTED DIE WITH VARYING SLOTS IMPINGE FIBERS wITH AIRHIGH VELOCITY PULSED HEATED GAS COANDA EXPAND GAS COOL FIBERS COLLECTFIBRIDS AND Fl LAM ENTS FIG. I

PATENTEU APR 23 m4 SHEET 2 [1P4 FIG. 3

FIG.

FIG. 5

APPARATUS FOR PRODUCING STRONG AND HIGHLY OPAQUE RANDOM .FIBROUS WEBSBACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates generally to nonwoven fibrous webs. More particularly, theinvention relates-to extrusion apparatus that produce webs characterizedby high Strength and opacity.

2. Description of the Prior Art It is known to form fibrous webs bydrawing fibers from an extruder and depositing them on a moving carriersuch as a rotating screen or moving wire. It is further known to performthe drawing step by impinging a current of air against the fibers. U. S.Pat. No. 3,509,009 to I-Iartmann, for example, discusses such methodsand apparatus for carrying them out including extruder dies formed bygrooves in wedges which are placed together so that the grooves arejuxtaposed. As

the polymer is extruded, the mass is blown into fine fibers through theuse of aircurrents and forms a web on a moving carrier. This patentteaches, however, that it is preferred to draw continuous filaments byentraining them in a gas stream directed into the filament path anddepositing them on a foraminous support.

It is also known and disclosed in U. S. Pat. No. 3,488,819 to Jackson,for example, that the Coanda effect of a divergent nozzle may beutilized to oscillate an airjet causing an entrained yarn to bedeposited on a carrier in continuous crosswise movement. In addition,the prior art contains teachings of the use of dies for extrusion havingvarious shapes and configurations of spinneret holes; such aredisclosed, for example, in U. S. Pat. Nos. 3,249,669 and 3,528,129.

SUMMARY OF THE INVENTION The present invention has as a primary objectto provide improvements in apparatus for-producing nonwoven fibrous websof exceptional strength and' opacity for a given weight. Other objectsand advantages will be apparent upon reference to the drawings and tothe detailed description below. r

In accordance with the invention the improved webs are formed by heatinga thermoplastic polymer to produce an extrudable melt, extruding themelt through a slotted film die having slots of correspondingly varyinglength and cross section, drawing the melt into fibers by impinging agas at the melt temperature at an angle to the extruding polymer,cooling the gas and fibers by al lowing the gas to expand therebycausing the fibers to break up in varying lengths, and collecting thebroken fibers into a web. In a preferred embodiment the apparatusincludes means for providing a Coanda effect and angular deposition inan interlocking crosswise wave pattern.

BRIEFDESCRIPTION OF THE DRAWINGS showing means for provid- FIG. 8schematically illustrates aformed web portion.

DETAILED DESCRIPTION OF THE INVENTION The apparatus of the inventionwill now be described in detail with particular reference to thepreferred embodiments illustrated in the drawings.

Turning to FIG. 1, the process involves first the selection of asuitable thermoplastic polymer. Many such polymers capable of extrusionin fiber form are avail-' able, and the particular one to be utilized isa matter of choice based usually on considerations such as cost, melttemperature, fiber properties, and extrusion properties, as well asdesired web characteristics. Examples of polymers contemplated for usein the invention include the following: polyolefins, nylons, polyesters,and

polyacetals. However, it will be recognized that other I slots withcorrespondingly varying slot length and cross sectional area dimensions.The phrase correspondingly varying" is used to indicate that the lengthand cross section area dimensions must vary in the same mannerl 'lhatis, the larger the slot length, the larger will be its cross sectionalarea. In general, 'each slot is preferably of uniform dimensions whichare different from those of adjacent slots..The fibers emerging from thedie are impinged by high velocity air at about, the melt temperaturedirected at an angle of from about 10 to 45 to the fiber orientationdirection. The gas is thereafter allowed to expand rapidly causingcooling and fracturing of many of the fibers into a mixture of fibrids,short fibers and filaments of widely varying di mensions. In, the termsof this invention the words frbrids, short fibers and filaments aredefined as follows: filaments aresubstantially continous strandsgenerally of l to 20 denier, short fibers are discontinuous strands Viinch to 3 inches inlength andgenerally of V: to 3 denier, and fibridsare short strands of less than V4 inch in length and generally less thanA denier. This mixture is collected on a moving carrier such as a screenor the like and forms a strong, opaque web which may be removed andwound into rolls with apparatus conventionally used for such purposes.By

opaque" it is meant that the web has high opacity for As indicated bythe flow diagram, FIG. I, an alternative embodiment includes the step ofusing a Coanda arrangement to direct the fibers. across the receivingcarrier. Pulsed air may be used to cause the fibers to deposit in wavypatterns across the carrier surface. This manner of deposition increasesthe interlocking nature of the random pattern of thefiber-filament-fibrid mixture thus augmenting strength and opacity.

Turning nowto FIGS. 2 to 5, the invention willbe described in terms ofthe embodiment illustrated therein.

where particularly suitable for use with polyolefins, e.g.,polypropylene, and results in a web having the previously I mentionedadvantages of strength, opacity, and gross uniformity of formation. Ofcourse, the particular dimensionsselected involve largely a matter ofchoice depending upon the desired fiber characteristics in the web andwill, in some cases, differ widely from the example recited. Inparticular, the specific slot crosssection shape need not be as shownsince it is recognized that the filaments, fibers, and fibrids will tendto become round in cross-section as cooling takes place.

It is only critical that the slots 20 exhibit significant variation inboth length and cross-sectional area across the lip 16 of die 10. Theratio of length to cross section is preferably selected for each polymerto result in generally equal shear stresses for each slot. Thus, theconditions are such that in all cases,

Pa=sCL P die pressure, psi;

C slot cross sectional circumference, in.

s shear stress, lbs./in.

a slot cross section, in.

L slot length, in. thus, s/P constant= a/CL for each slot. While thecircumference of the slots depends on the cross sectional sharia. sheaths me. h p is. s fer a." slots, it

will vary in proportion to V5. Hence, it is preferred that theindividual slot lengths be proportioned'according to V5 to maintain aconstant value of 'a/CL. By keeping the shear stress generally equal inall slots, flow through each one'is assured. The particular slot crosssection shape depends upon the desired properties and end use. It may beround, rectangular, or triangular, for example. A dog-bone shape may beused to produce an effect of high luster.

. The number of teeth 18 per inch of width will simi-' larly bedependent upon desired fiber properties such as denier as well as webproperties such as basis weight. However, it is contemplated that thetypical ranges for these dimensions for producing a web with paper-likestrength and opacity will be as follows: length of groove (L,L) 0.008 to0.5 inch; cross sectional area of groove 0.0001 in. to 0.0025 in.,preferably 0.0002 in. to

0.0015 in.; and number of grooves from about 5 to 100 within chamber22.As shown in cross-section the housing. includesdual air manifolds 24which direct air flow frpm a source "at elevated temperature (not shown)down the beveled edges of die 10 to lip 16 and out through nozzle 26. Inaccordance with the invention in a preferred embodiment, nozzle 26includes an outlet 28 having a nearly symmetrical Coanda configurationwhich has been exaggerated in the drawing for illustrative purposes. Byavoiding complete symmetry it is possible to produce higher stability onone side of nozzle 26 to which the fibers will naturally be drawn asthey are formed; in the illustrated case the preferred side will be side30 which has a slightly smaller, by about 1 to 10 per cent, for example,radius of curvature when compared to the radius of side 36. Channel 32is pro vided in side 30 and connected to a source (not shown) of pulsedair. Such sources are well known and, in

longer fibers. The upper limit of 2 1,600 yards per minute has beenselected as representing sonic velocity. It is recognized; however, thatwith special orifice designs, higher rates are obtainable; whenavailable, these higher air rates may also be utilized with the presentinvention.

w The samba the fibers warlidaamriany tend to take due to the slightdissymetryof the Coanda configuration is illustrated by arrow A of FIG.6,. As the air expands, rapid cooling of the polymer filaments 34 takesplace causing them to separate into the various lengths forfilaments,short fibers, and fibrids. Air under pressure of about 0.1 to l psig,for example, is pulsed through channel 32 and, when air is flowing inthe channel, causes displacement of the fiber flow towards side 36.Channel 32 is preferably positioned so that the excited pulsed air movesgenerally at a tangent to the outlet-curve of side 36 on nozzle 26, thustending to urge the fibers in that direction as well, as indicated byarrow B. When air is not flowing in channel 32, the fibers will tend toresume the original flow direction A. The result, indicated by arrow C,is a generally sine wave deposition path in the direction of travel ofcarrier 38 with afrequency that depends upon the pulsing frequency. Thisfrequency maybe, for example, in the range of 2 to 600 cycles persecond. In a critically designed system, for example, an oscillatingdiaphragm of an earphone placed in channel 32 could beused to replacethe air pulses and induce vibrations and fiber deposition at'a desiredfrequency.

deposited by nozzle 26. As shown, it is preferred thatdie 10 bedisposed'angularly with respect to the lateral direction of movement ofsurface 38. The angle, a, se lected is not critical and preferably mayvary, for exarnq In operation, the polymer is extruded through die 10pie, between 30 and 60. It has been found that this angular depositionin combination with the controlled Coanda effect results in wavy motionof the fibers in the direction of movement of surface 38. The fibers arecaught in this interlocking pattern as a random mixture of orientedfilaments, short fibers and fibrids. The rapid cooling caused by thenozzle expansion prevents deorientation of the fiber molecules andshrinking of the fiber ends which has previously resulted in balling andlumping producing poor formation characteristics.

FIG. 8 schematically illustrates the web 40 formed on surface 38 whichmay be subsequently treated by methods and for purposes that are wellknown. For example calendaring may be used, and the resulting sheet willdisplay increased stength and density due to the fibridsintermixedbetween the fibers.,These fibrids also increase the totalinner surface area thus providing in- Y I forcing material for tapes.Other uses for specific combinations will be apparent to those skilledin this art.

In summary, the invention comprises an improved apparatus for use in amethodfor producing strong, opaque fibrous webs including the steps ofextruding a polymer through a die having slotsof corresponding varyingdimensions and impinging the fibers at an angle with heated, highvelocity air. The fibers are cooled by rapid air expansion and collectedon a carrier to form a web. A controlled Coanda may-be used and thefibers deposited at an angle with respect to the carrier in preferredarangements. The apparatus of the invention includes a die having a lipwith slots of correspondingly varying dimensions as well as means forapplying a con trolled Coanda effect and angular deposition in thepreferred embodiments.

, I claim: i

LApparatus for producing a high strength opaque web of random filaments,fibers. and fibrids; from I a thermoplastic polymer comprising, incombination,

a. means for providing the polymer in extrudable form; b. an extruderadapted to receive said polymer and having a slotted die with slots ofcorrespondingly varying length and cross sectional area dimensionsselected to result in generally equal shear stresses for each slot;

c. means for providing high velocity air at about the extrusiontemperature of said. polymer;

d. conduits for said air causing impingement with-the extruding polymeras it emerges from the extruder, said impingement being from opposingsides at an angle of from about l0 to 45 to the fiber'direction andtending to draw and orient said emerging fibers; j

e. adiverging nozzle configuration for rapidly expanding and coolingsaid air and-extruding polymers; and

f. means for collecting said extruding polymer as it is formed intofilaments and fibrids.

2. Apparatus of claim 1 wherein said extruder and die are disposedangularly with respect to the collecting means. i

3. Apparatus of claim 1 further including means for providing a Coandaeffect wherein said diverging nozzle has a nearly symmetrical Coandaconfiguration and pulsing means are provided to disturb the Coanda cffeet with the result that said extruding polymer is deposited on saidcollecting means in aninterlocking wave pattern. 7

4..Apparatus of claim 1 wherein said die slots vary regularly in length.inthe range of from about 0.008

inch to 0.500 inch andin cross sectional area in the range of from0.0001 in. to about 0.0025in. with from five to slots per inch providinggenerally equal shear stresses ineach slot.- n

5. The apparatus of claim 3 wherein said pulsing means is a conduit forpulsed air at a pressure slightly 7. The apparatus of claim 5' whereinsaid pulsing means includes an oscillating diaphragm.

' a: is =0: s g

1. Apparatus for producing a high strength opaque web of randomfilaments, fibers and fibrids, from a thermoplastic polymer comprising,in combination, a. means for providing the polymer in extrudable form;b. an extruder adapted to receive said polymer and having a slotted diewith slots of correspondingly varying length and cross sectional areadimensions selected to result in generally equal shear stresses for eachslot; c. means for providing high velocity air at about the extrusiontemperature of said polymer; d. conduits for said air causingimpingement with the extruding polymer as it emerges from the extruder,said impingement being from opposing sides at an angle of from about 10*to 45* to the fiber direction and tending to draw and orient saidemerging fibers; e. a diverging nozzle configuration for rapidlyexpanding and cooling said air and extruding polymers; and f. means forcollecting said extruding polymer as it is formed into filaments andfibrids.
 2. Apparatus of claim 1 wherein said extruder and die aredisposed angularly with respect to the collecting means.
 3. Apparatus ofclaim 1 further including means for providing a Coanda effect whereinsaid diverging nozzle has a nearly symmetrical Coanda configuration andpulsing means are provided to disturb the Coanda effect with the resultthat said extruding polymer is deposited on said collecting means in aninterlocking wave pattern.
 4. Apparatus of claim 1 wherein said dieslots vary regularly in length in the range of from about 0.008 inch to0.500 inch and in cross sectional area in the range of from 0.0001 in.2to about 0.0025 in.2 with from five to 100 slots per inch providinggenerally equal shear stresses in each slot.
 5. The apparatus of claim 3wherein said pulsing means is a conduit for pulsed air at a pressureslightly higher than said heated air.
 6. The apparatus of claim 5wherein said conduit provides air pulsed at a frequency of from 2 toabout 600 cycles per second which is directed at an angle tangent to theoutlet curve side of said diverging nozzle.
 7. The apparatus of claim 5wherein said pulsing means includes an oscillating diaphragm.